Exam 1 - Acid Base Management Flashcards
Acid
Ability to donate H+
Base
Ability to accept H+
pH
pH = -log[H+]
Dissociation curve shifts left if…
- Temp decreases
- Energy decreases
- Dissociation decreases
- concentration of ions decrease - pH increases
pH of neutrality
pH produced when water dissociates
- pH = 7.0 at 27C
Henry’s Law
Dissolved (total) gas = Solubility x Partial Pressure
Solubility of CO2 @ 37C
0.06 mls CO2 / 100 mls blood / mmHg
Solubility of O2 @ 37C
0.003 mls O2 / 100 mls blood / mmHg
Partial pressure of air phase vs. liquid phase
Equal at equilibrium
Forms of CO2 from tissue to lungs
- Dissolved
- Bound to hemoglobin (limited ceiling)
- As HCO3
~ conversion happens in RBC VI’s carbonic anhydrase
~ unlimited ceiling - All add up to total CO2 content
Haldene effect
Venous blood hemoglobin has greater affinity to CO2 due to lower O2 content
Solubility vs Temperature
If temp goes up….solubility of gas goes down
How to change pH of blood in CLOSED system
- Change total CO2 content (temp constant)
- Change temp (total CO2 constant)
- Change temp AND total CO2 content
CO2 vs pH
If CO2 goes up….pH goes down
Increasing total CO2 content by adding CO2
- pCO2 increases
- pH decreases
- visa versa
- Assuming no change in temp/ solubility / disassociation equilibrium
Increasing temperature affect
- Drive disassociation equilibrium to the right
- the reason for the change in pH - [H+] increases
- pH decreases
- visa versa
- Assuming no change in total CO2
- pCO2 changes because solubility changes…but NO change in pH
If temperature increases in closed system…
- Gas solubility DECREASES
- Gas partial pressure INCREASES
- Gas pressure is GREATER than starting pressure
- Equilibrium shifts RIGHT
- pH DECREASES
If Temperature decreases in a closed system
- Gas solubility INCREASES
- Gas partial pressure DECREASES
- Gas pressure is LESS than starting pressure
- Equilibrium shifts LEFT
- pH INCREASES
What is needed to draw a sample
- Waste syringe (for venous draw)
- Sample syringe
- Cup of ice (if going to lab)
- Draw from patient / ECC / remove air from sample
Blood gas determination in lab
- Analyzer warms sample to 37C
- determines pO2, pCO2, and pH at 37C
- At 37C…solubility decreases…pO2/pCO2 will be artificially high
- Pushes HOH equation right so pH will be artificially low at 37C
Uncorrected gases
- Report gas values at 37C
Corrected gases
- Gas values measured at 37C BUT corrected back to actual body temperature
pH change per degree
0.0147 drop in pH per increase in degree C
ALWAYS use temperature corrected values for…
pO2
3 H+ equations
HOH OH- + H+
CO2 + H2O H2CO3 HCO3- + H+
Protein+ Protein + H+
- A change in one of these will have an effect on the other two
- Increasing H+ will shift others left
3 big buffer players
- Bicarb
- Protein
- Phosphate
- 16:1 OH:H buffer ratio despite changes in temperature
- Main blood buffering system is imidazole side chain of histidine
- accounts for most of pH change due to changes in temp
Alpha and pH stat only affect…
- pCO2 and pH
- Arterial pO2 MUST be temperature corrected
Normal pCO2
40 mmHg
Normal pH
7.4 at 37C
Why does pH change when CO2 is added
Increase in H+ ions
CO2 + H2O H2CO3 HCO3- + H+
Alpha stat
- Total CO2 content held constant with change in temp
- Values will drift naturally as temp drops
~due to solubility changes and Imidazole disassociation shift - What happens to our bodies on daily basis
~between pulmonary caps and systemic caps - Alpha ratio held constant
pH stat
- Hold pCO2 and pH at constant level with change in temp
- What happens in reptiles / cold blooded animals
- Total CO2 content MUST increase to maintain levels
~add CO2 as temp decreases and remove CO2 as temp increase
~ must blow off CO2 when rewarming - Alpha ratio changes as CO2 content increases
pH vs body temperature
- vary inversely
- in body… temperature (and pH) will vary from organ to organ
Advantages of constant CO2 content (alpha stat)
- constant pH across cell membranes
- constant ratio of - to + charge on proteins
- allows enzymes to work at optimal level regardless of temp
- Keeps metabolic activity up even at low temps
Consequences of pH stat during Hypothermia
- Blood more acidic
- ion gradient across cell membrane changes
- acidity alters metabolic activity (not always bad in surgery)
- cells do not maintain neutrality (bad)
- added CO2 influences blood flow regulation…big in brain
~ biggest effect
Consequences of alpha stat during hypothermia
- constant charge on histamine side chain
- ion gradient across cell is constant
- enzyme functions maintained (metabolic activity higher in surgery)
- cells maintain neutrality
- auto-regulation of blood flow maintained…based on metabolic need
pH stat and tissue oxygenation
- shifts oxy-hemo curve right which offsets left shift due to hypothermia
CO2 as a vasodilator
- pH stat: increases / maldistributes blood flow
- alpha stat: improve peripheral perfusion because we maintain auto regulation
Cerebral blood flow in pH stat
- normally tightly auto regulated
- below 20C….lose autoregulation (for both stats)
- pressure dependent…greater blood flow
- improved cooling (due to more flow)
- Luxury flow
- Risk of embolic injury, high brain pressure, edema
~ more so in adults
Luxury flow
Flow to brain is higher than metabolic need of brain
Cerebral flow in alpha stat
- lower blood flow - based on metabolic need
- risk of adverse flow distribution, ischemic injury
~ more so in infants/peds
Best strategies
- alpha: regular adult bypass if mild to moderate
- pH stat: infants
- Combo - pH to cool / alpha to warm: profound arrest w/ adults
Managing pH stat
- keep pH constant
- keep temp corrected pCO2 at 40
- keep temp corrected pH at 7.4
Normal bicarb
22-26
Normal SvO2
70
Normal pO2
150-250 on pump
Managing alpha stat
- keep total CO2 constant…allow pH to change
- target values change with temp
- use NOT corrected values for pCO2 and pH and keep normal
- can compare patient core temp to corrected pCO2 and keep same
If brain oxymetry says need more flow….
- Give more CO2
